1. The problem statement, all variables and given/known data Can a single mode fiber become multimode if the wavelength of the light passed through it is small enough? I put a HeNe laser (633nm) through what I thought was a single mode fiber. If it's supposed to operate at 1300nm, then would a low wavelength of 633nm cause multiple modes in the single mode fiber? 2. Relevant equations number of modes = operational wavelength / wavelength used 3. The attempt at a solution number of modes = 1300nm/633nm = (approx.) 2 modes? I'm not sure why my image isn't showing a uniform mode / pattern.